Psychomotor stimulants can cause humans to report being more alert and more attentive to their surroundings; their attractiveness to recreational users may depend partly upon these effects. Some stimulants are prescribed to deal with the distractability and short attention span seen in children diagnosed as having an attention-deficit hyperactivity disorder. These facts implicate discriminative control by environmental stimuli as an important aspect of behavior targeted by the drugs. This is a proposal to study how various psychomotor stimulants, including cocaine, d-amphetamine, methylphenidate and pipradrol, affect two aspects of discriminative control: stimulus control enhancement and stimulus control modulation. Two different multiple schedules of reinforcement will be used. On one, based upon the fixed-consecutive-number schedule, the subject must make a specified minimum number of consecutive responses to quality for reinforcement. Manipulation of the stimuli present when the number requirement is satisfied permits study of both whether the drugs enhance the strength of stimulus control at that time and whether the addition of external discriminative stimuli modifies how the drugs affect behavior. Analogous studies will be performed with a multiple schedule based upon the fixed-minimum-interval schedule, on which a minimum time must be spent without responding in order to satisfy the reinforcement requirement. Information generated by these studies could eventually aid the design of more effective therapeutic agents for attention-deficit hyperactivity disorders as well as the development of drug abuse treatment regimens that more adequately minimize drug-produced disruption of behavior.